As shown in FIG. 10, in a conventional diaphragm type carburetor, a constant pressure fuel supply mechanism B having a fuel metering chamber 13 defined by a diaphragm 9 is provided in the lower part of a carburetor body 32. A passage 2 is communicated with the fuel metering chamber 13 and with a fuel chamber 27 via a fuel adjusting needle valve 31. A plurality of low speed fuel nozzle holes 28 open into an air intake passage 30 upstream of a throttle valve 24 when it is in its idle position, and a single pilot hole 28a opens to the air intake passage 30 on the downstream side of the throttle valve 24. Further, a high speed fuel nozzle hole 29 is open to a venturi portion 30a of the air intake passage 30 and communicates with the fuel metering chamber 13 via a fuel jet 5, a passage 3, and a check valve 29a. 
In the above-described diaphragm type carburetor, fuel in the fuel metering chamber 13 is directly taken into the air intake passage 30 from the pilot fuel nozzle holes 28a. Fuel is not always sufficiently mixed with air in the air intake passage 30, and particularly during idle operation of the engine, fuel from the low speed fuel nozzle hole 28 may collect on the throttle valve 24 to the inner peripheral wall of the air intake passage 30, and the mixture of fuel and air is not always supplied to the engine in a stabilized manner. Particularly, when fuel that had collected on the inner wall of the air intake passage 30 suddenly enters the airflow through the air intake passage 30 due, for example, to the tilting of the carburetor body, a richer than desired fuel and air mixture is supplied to the engine momentarily, and may adversely affect idle operation of the engine. In extreme cases, the fuel and air mixture may be so rich that the engine stalls, and immediate re-starting of the engine may be difficult.
The carburetor as shown in FIG. 11, can have the same problem. As shown in FIG. 11, the fuel metering chamber 13 is communicated with the fuel chamber 27 via a fuel jet 4. The passage 2, in the idle running of the engine, is taken into a chamber 27 on the upstream side of the throttle valve 24 via the low speed fuel nozzle holes 28. Simultaneously, the fuel in the fuel chamber 27 is fed to a pilot passage 28a (via a fuel adjusting needle valve 31) which opens into the air intake passage downstream of the throttle valve 24 in its idle position. Fuel taken from the low speed fuel nozzle holes 28 and the pilot passage 28a is to be mixed with air in the air intake passage 30, and may not obtain sufficient atomization of fuel.
Further, in the conventional float type carburetor, as shown in FIG. 12, an air passage 42 substantially parallel to the air intake passage 30 and having a first end communicated with the air intake passage 30 is provided in carburetor body 32. A low speed fuel supply pipe 47 extending from a float chamber 57 is connected to the air passage 42 by low speed fuel nozzle holes 28 open to a fuel chamber 27 and to the air intake passage 30 upstream of a throttle valve 24 when in its idle position. The other end of the air passage 42 is connected to a valve chamber 34 of a fuel adjusting needle valve 31, and a pilot fuel nozzle hole 28a is open to the air intake passage 30 downstream of the throttle valve 24 when in its idle position. In the above-described float type carburetor, since fuel from the low speed fuel supply pipe 47 is mixed with air during fuel flows to the low speed fuel nozzle holes 28 and the pilot fuel nozzle hole 28a, atomization of fuel is accelerated as compared with that shown in FIGS. 10 and 11, but since the air flow in the air passage 42 is weak, atomization of fuel may not be sufficient.